There is an established need for transparent conducting materials for use as transparent electrodes, coatings and films. Such transparent materials are required in a variety of device applications, including for example the electrodes for liquid crystal displays and transparent conductive coatings for antistatic applications. Other applications include, for example, use as electrodes on polymer (or composite) objects to enable electroplating and as conductive transparent films for packaging and electrically shielding electronic goods.
Only a few materials exhibit the combination of relatively high electrical conductivity and optical transparency. The most widely used examples are mixed inorganic oxide materials, for example indium/tin oxide (ITO) and other related oxides.
Although ITO has adequate properties for many uses in technology, ITO also has many disadvantages, including the following:
(i) High vacuum technology (for example sputtering) is required for application of transparent conducting ITO or related mixed oxides onto a substrate. This vacuum technology is expensive, since it requires major capital investment for the equipment needed for application. Furthermore, application, for example by sputtering, onto curved or complex surfaces is difficult. PA1 (ii) The precise chemical stoichiometry and morphology required for transparent conducting ITO or related mixed oxides is difficult to achieve and difficult to control. As a result, the preparation of transparent conducting ITO or related mixed oxide films is often treated as a trade secret. Thus, the routine manufacture of transparent conducting ITO or related mixed oxide films for transparent electrode applications requires precise and detailed process control. PA1 (iii) Transparent conducting ITO or related mixed oxide films are brittle. Thus, when applied onto flexible substrates (for example onto free standing polymer films as substrates), the coated substrate is delicate and the transparent electrode is easily shattered. PA1 (iv) Patterning of the transparent conducting ITO or related mixed oxides requires etching the insoluble ITO or related mixed oxide material. Although possible, relatively highly corrosive etching solutions are required. PA1 (i) Because the transparent conducting body, coating or film is a stable soluble polymer blend, the conducting transparent film can be applied by casting from solution (for example, spin-casting, drop-casting, etc). This can be carried out in ambient atmosphere with no need for vacuum technology. PA1 (ii) Since the transparent conductor is cast onto the substrate directly from solution, said material can be cast onto complex, curved surfaces. PA1 (iii) The precise chemical stoichiometry is pre-determined by the concentrations of the conducting polyaniline complex and the PMMA in the solution used for casting the electrode film. Thus, the manufacture of transparent conducting films for electrode applications and the like is routine. PA1 (iii) Transparent conducting materials fabricated from the conducting polyaniline complex/PMMA polyblends are flexible and mechanically robust. Thus, when applied onto flexible support surfaces (for example onto free standing polymer films) the coated surfaces are robust. PA1 (iv) Since transparent conducting films fabricated from the conducting polyaniline complex/polymer polyblends are re-soluble in common organic solvents, transparent electrodes and other conductor forms can be patterned using photo-lithographic techniques; said techniques being, for example, routinely used in the semiconductor industry.
Thus, there is a need for materials which are both electrically conducting and optically transparent, and which can be applied or fabricated directly from fluid phases at easy-to-obtain conditions. There is also a need for conductive devices and films which are mechanically robust and have specific and easily controlled compositions.
There exists prior art in the area of transparent conducting coatings made from conductive polymers:
(1) Shacklette et al (U.S. Pat. No. 4,963,206, Oct. 16, 1990) applied a conductive polyaniline film onto Aclar by exposing the Aclar film to a mixture of aniline tosylate and ammonium persulfate in an aqueous solution of tosic acid. Thus the conductive polyaniline film was polymerized in situ onto the substrate.
(2) Fukunishi et al (JP application no. 63145326, Jun. 17, 1988) used similar techniques to prepare polymer composites by in situ polymerization of pyrrole and aniline.
(3) Takahashi et al (JP application no. 63268733) prepared thin semitransparent films by electrolytic polymerization.
(4) Sakai et al (JP application no. 63215772, Sep. 8, 1988) manufactured conductive polymer compositions by polymerizing monomers capable of forming anionic polymer electrolytes in the presence of polymers of .pi.-conjugated structure. Transparent thin films were deposited electrolytically.
There is no known prior art in which optical quality transparent conducting polymer films have been Acast directly from a fluid phase (melt or solution) in the conductive form (without need for subsequent doping) as the pure conductive polymer or as polyblends containing the conductive polymer.
The present invention employs polyanilines as conductive polymers. The following is a general summary of art concerning these materials.
Kobayashi Tetsuhiko et al., J. Electroanal Chem., "Electrochemical Reactions Concerned With electrochromism of Polyaniline Film-Coated Electrodes," 177 (1984) 281-291, describes various experiments in which spectroelectro-chemical measurement of a polyaniline film coated electrode were made. French Patent No. 1,519,729, French Patent of Addition 94,536; U.K. Patent No. 1,216,549; "Direct Current Conductivity of Polyaniline Sulfate," M. Donomedoff, F. Kautier-Cristojini, R. ReSur-vail, M. Jozefowicz, L. T. Yu, and R. Buyer, J. Chim. Phys. Physicohim. Brol., 68, 1055 (1971); "Continuous Current Conductivity of Macromolecular Materials," L. T. Yu, M. Jozefowicz, and R. Buyer, Chim. Macromol., 1,469 (1970); "Polyaniline Based Filmogenic Organic Conductive Polymers,"-D. LaBarre and M. Jozefowicz, C.R. Read Sci., Ser. C, 269, 964 (1969); "Recently Discovered Properties of Semiconducting Polymers," M. Jozefowicz, L. T. Yu, J. Perichon, and R. Buyer, J. Polym. Sci., Part C, 22, 1187 (1967); "Electrochemical Properties of Polyaniline Sulfates," F. Cristojini, R.-De Surville, and M. Jozefowicz, Cr. Read. Sci., Ser. C, 268, 1346 (1979); "Electrochemical Cells Using Protolytic Organic Semiconductors," R. De Surville, M. Jozefowicz, L. T. Yu, J. Perichon, R. Buvet, Electrochem. Ditn. 13, 1451 (1968); "Oligomers and Polymers Produced by Oxidation of Aromatic Amines," R. De Surville, M. Jozefowicz, and R. Buvet, Ann. Chem. (Paris), 2, 5 (1967) "Experimental Study of the Direct Current Conductivity of Macromolecular Compound," L. T. Yu, M. Borredon, M. Jozefowicz, G. Belorgey, and R. Buyer, J. Polym. Sci. Polym. Symp., 16, 2931 (1967); "Conductivity and Chemical Properties of Oligomeric Polyaniline," M. Jozefowicz, L. T. Yu, G. Belorgey, and R. Buyer, J. Polym. Sci., Polym. Symp., 16, 2934 (1967); "Products of the Catalytic Oxidation of Aromatic Amines," R. De Surville, M. Jozefowicz, and R. Buyer, Ann. Chem. (Paris), 2, 149 (1967); "Conductivity and Chemical Composition of Macromolecular Semiconductors," Rev. Gen. Electr., 75 1014 (1966); "Relation Between the Chemical and Electrochemical Properties of Macromolecular Semiconductors," M. Jozefowicz and L. T. Yu, Rev. Gen. Electr., 75, 1008 (1966); "Preparation, Chemical Properties, and Electrical Conductivity of Poly-N-Alkyl Anilines in the Solid State," D. Muller and M. Jozefowicz, Bull. Soc. Chem. Fr., 4087 (1972).
U.S. Pat. Nos. 3,963,498 and 4,025,463 describe oligomeric polyanilines and substituted polyanilines having not more than 8 aniline repeat units which are described as being soluble in certain organic solvents and which are described as being useful in the formation of semiconductors compositions. European Patent No. 0017717 is an apparent improvement in the compositions of U.S. Pat. Nos. 3,963,498 and 4,025,463 and states that the polyaniline can be formed into a latex composite through use of the oligomers of polyaniline and a suitable binder polymer.
High molecular weight polyaniline has emerged as one of the more promising conducting polymers, because of its excellent chemical stability combined with respectable levels of electrical conductivity of the doped or protonated material. Processing of polyaniline high polymers into useful objects and devices, however, has been problematic. Melt processing is not possible, since the polymer decomposes at temperatures below a softening or melting point. In addition, major difficulties have been encountered in attempts to dissolve the high molecular weight polymer.
Recently, it was demonstrated that polyaniline, in either the conducting emeraldine salt form or the insulating emeraldine base form, can be processed from solution in certain strong acids to form useful articles (such as oriented fibers, tapes and the like). By solution processing from these strong acids, it is possible to form composites, or polyblends of polyaniline with other polymers (for example polyamides, aromatic polyamides (aramids), etc.) which are soluble in certain strong acids and thereby to make useful articles. "Electrically Conductive Fibers of Polyaniline Spun from Solutions in Concentrated Sulfuric Acid," A. Andreatta, Y. Cao, J. C. Chiang, A. J. Heeger and P. Smith, Synth. Met., 26, 383 (1988); "X-Ray Diffraction of Polyaniline," Y. Moon, Y. Cao, P. Smith and A. J. Heeger, Polymer Communications, 30, 196 (1989); "Influence of the Chemical Polymerization Conditions on the Properties of Polyaniline," Y. Cao, A. Andreatta, A. J. Heeger and P. Smith, Polymer, 30, 2305 (1990); "Magnetic Susceptibility of Crystalline Polyaniline," C. Fite, Y. Cao and A. J. Heeger, Sol. State Commun., 70, 245 (1989); "Spectroscopy and Transient Photoconductivity of Partially Crystalline Polyaniline," S. D. Phillips, G. Yu, Y. Cao, and A. J. Heeger, Phys. Rev. B 39, 10702 (1989); "Spectroscopic Studies of Polyaniline in Solution and in the Solid State," Y. Cao and A. J. Heeger, Synth. Met. 32, 263, (1989); "Magnetic Susceptibility of One-Dimensional Chains in Solution," C. Fite, Y. Cao and A. J. Heeger, Solid State Commun., 73, 607 1990); "Electrically Conductive Polyblend Fibers of Polyaniline and Poly(p-phenylene terephthalamide)," A. Andreatta, A. J. Heeger and P. Smith, Polymer Communications, 31, 275 (1990); "Polyaniline Processed From Sulfuric Acid and in Solution in Sulfuric Acid: Electrical, Optical and Magnetic Properties," Y. Cao, P. Smith and A. J. Heeger in Conjugated Polymeric Materials: Opportunities in Electronics, Opto-electronics, and Molecular Electronics, ed. J. L. Bredas and R. R. Chance (Kluwer Academic Publishers, The Netherlands, 1990).
U.S. Pat. No. 4,983,322 describes solutions and plasticized compositions of electrically conductive substituted and unsubstituted polyanilines and methods of forming such solutions or compositions and use of same to form conductive articles. The polyaniline materials were made soluble by the addition of an oxidizing agent such as FeCl.sub.3. Since the resulting compounds are charge transfer salts, highly polar solvents were required; specifically solvents were needed with dielectric constants equal to or greater than 25 and with dipole moments equal to or greater than 3.25.
Starting with the insulating emeraldine base form, polyaniline can be rendered conducting through two independent doping routes:
(i) Oxidation either electrochemically (by means of an electrochemical charge transfer reaction) or chemically (by means of chemical reaction with an appropriate oxidizing agent such as FeCl.sub.3);
(ii) Protonation through acid-base chemistry by exposure to protonic acids (for example, in aqueous environment with pH less than 2-3). (1) `Polyaniline`: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime by J.-C. Chiang and Alan G. MacDiarmid, Synthetic Metals 13 193 (1986). (2) A Two-Dimensional-Surface `State` Diagram for Polyaniline by W. R. Salaneck, I. Lundstrom, W.-S Huang and A. G. MacDiarmid, Synthetic Metals 13, 297 (1986).
These two different routes lead to distinctly different final states. In (i), the oxidation causes a change in the total number of .pi.-electrons on the conjugated chain and thereby renders it conductive. In (ii), there is no change in the number of electrons; the material is rendered electrically conductive by protonation of the imine nitrogen sites.
A need exists for techniques and materials to facilitate the fabrication of shaped transparent conductive polyaniline articles, especially articles made from bulk material (conductive polyanilines and/or composites, or polyblends of conductive polyaniline with other polymers) and films, fibers and coatings.